1. Field of the Invention
The present invention relates to an electric double layer capacitor.
This application is based on Japanese Patent Application No. Hei 9-295934, the contents of which are incorporated herein by reference.
2. Description of Related Art
An example of a conventional electric double layer capacitor is shown in FIGS. 7 and 8. FIG. 7 is a perspective view of the example of the conventional electric double layer capacitor. FIG. 8 is a cross sectional view showing a structure of the unit cell in the conventional electric double layer capacitor shown in FIG. 7.
In FIG. 8, a solid type activated carbon, for example, an active carbon/polyacene system material, forms a polarizing electrode 1.
A collector 2, which is made of a rubber or a plastic containing conductive carbon, is pressed onto the polarizing electrode 1. A pair of the polarizing electrodes 1 face each other on both sides of a porous separator 4. An electrolyte solution 3 is sealed with both a frame-form gasket 5 and collector 2 shown in FIG. 8. An assembly of the polarizing electrodes 1, the collector 2, the electrolyte solution 3, separator 4, and the gasket 5 makes a unit cell 6.
Since the withstanding voltage of an electric double layer capacitor depends on the electrolysis voltage of the electrolyte solution 3, a plurality of unit cells 6 are connected in series according to a desired withstanding voltage. Furthermore, the unit cells 6 are held between terminal electrodes 8 by pressing loading plates 7 on the external sides of the terminal electrodes as shown in FIG. 7 in order to reduce the degree of contact resistance. A cell stack 9 consists of a plurality of unit cells.
In recent years, several properties of electric double layer capacitors have been improved, such as increased capacity of the capacitor and reduced equivalent series resistance (which is hereinafter called "ESR") by using the polarizing electrodes 1, and thereby, new uses for these capacitors have been discovered, and practical applications are currently being studied. Electric double layer capacitors are used, for example, to form part of a power supply for driving a starter motor of an automobile in combination with a lead battery, or as an auxiliary power supply in combination with a solar battery or the like.
Electric double layer capacitors will probably be subjected to high temperature conditions, therefore, it is necessary to ensure their reliability under such conditions. At the same time, it is necessary to minimize their self-discharge.
In conventional electric double layer capacitors, if porosity of the separator 4 is reduced in order to improve their self discharge characteristics (which is hereinafter called "SD characteristics"), the amount of the electrolyte solution 3 which is retained in the unit cell 6 is reduced. If the capacitor with reduced amount of the electrolyte solution 3 is preserved under high temperature conditions, the amount of the electrolyte solution 3 held in the unit cell 6 will be gradually reduced by vaporization, that is to say, a "dry up phenomenon" occurs. Consequently, the electrostatic capacity of the capacitor will be reduced and the ESR will increase, which result in deteriotation of the reliability. Therefore, it is desirable to increase the porosity in the separator 4 as large as possible for retaining the electrolyte solution 3 as much as possible in the separator 4, in order to ensure reliability.
However, increased porosity of the separator 4 to ensure reliability will also increase the amount of the electrolyte solution 3 which is retained in the separator 4, and as a result, self discharge will tend to occur, and as a result, unfavorable SD characteristics will be encoutered. Therefore, from the point of view of SD characteristics, it is desirable that the porosity of the separator 4 should be as small as possible and the holding amount of the electrolyte solution 3 should be reduced.
As hereinabove described, the requirements for ensuring reliability under high temperature conditions are contrary to the requirements for improving the SD characteristics.
Furthermore, an electric double layer capacitor such as the capacitor shown in FIG. 9 is disclosed in Japanese Patent Application, First Publication, Hei 6-45191. The above application discloses that two porous sheets made of the same material are attached so as to form a separator. The above application is made so as to improve the mechanical strength of the porous separator to withstand pressure from both ends.
Japanese Patent Application, First Publication, Sho 57-97613 discloses a technique, similar to the above publication, to attach two sheets made of the same material so as to form the separator.
Furthermore, Japanese Patent Application, First Publication, Hei 4-338623 discloses an Example of attaching two sheets made of the same melt-plain nonwoven fabric for forming separators.
In the above-mentioned techniques for attaching two sheets made of the same material to each other, the increase in the amount of electrolyte solution held in the separator improves the reliability under high temperatures, but degrades the SD characteristics.
On the other hand, if two sheets with low porosity are attached to form a separator in order to improve the SD characteristics, the reliability under high temperatures is reduced.
Therefore, maintaining high reliability and improving the SD characteristics under high temperature conditions are contrary to each other, and both cannot be achieved at the same time.
Furthermore, Japanese Patent Application, First Publication, Hei 4-151816, discloses an electric double layer capacitor comprising the unit cell containing structure shown in FIG. 10.
In the technique recited in the above-mentioned application, a separator 4 has a structure wherein a high-hydrophilic fiber 4a based on acrylic resin is laminated on both sides of a porous separator main body 4b by thermocompressing the high-hydrophilic fibers 4a at thermocompression bonding portions 4c at intervals of 5 mm.
It is expected by using the above structure, to improve both the water retentivity of the electrolyte solution, and the reliability under high temperatures.
However, since the high-hydrophilic fiber 4a are laminated at thermocompression bonding portions 4c by thermocompression bonding, this requires strict control of the manufacturing conditions, for example, the temperature when the porous separator main body 4b is thermocompressed to the high-hydrophilic fibers 4a and the pressure for thermocompression bonding. Therefore, if the temperature or pressure for the thermocompression bonding is too high, the pores in the porous separator main body 4b may collapse or the high-hydrophilic fiber may be torn, and the tears may make pinholes. On the other hand, if the temperature or pressure is too low, thermocompression bonding is difficult. As a result, unit cells containing the electric double layer capacitor according to the above-mentioned application cannot be produced with high reliablity.
Additionally, at the thermocompression bonding portions 4c, the pores in the thermocompressed high-hydrophilic fiber 4a completely collapse. Therefore, it is impossible to prevent reduction of the water retentivity of the electrolyte solution, and the effect of improvement of reliability under high temperature is reduced.